1. Technical Field
The invention relates to a fibre coating method and apparatus in particular for coating optical fibres.
2. Related Art
It is known to protect optical fibres by laying them in ducts. One well-known approach to laying the fibres in ducts comprises fibre blowing (or “blown fibre”). European patent application no. 0108590, commonly assigned herewith describes a fibre blowing technique in which the fibre is blown into a duct on a cushion of air, relying on viscous drag to advance the fibre.
In order to further protect the fibres conventional resin coatings such as silicone coatings or UV cured acrylate polymer are known, providing protection from damage and micro cracks. It has been found that embedding glass microspheres or other particulate matter into a resin coating on the fibres such as an acrylate polymer provides particularly good results for blown fibre. The coating is applied to an optical fibre unit comprising either a single fibre or a bundle of fibre.
European patent application no. 0521710, commonly assigned herewith, describes an improved method of coating an optical fibre unit so as to obtain better viscous drag and lower friction in a blown fibre process, where particulate matter such as microspheres typically between 10 and 200 μm diameter are embedded in the resin coating. In the process, fibre is fed from a drum through a first resin curing system, a further resin coating is applied, the fibre unit passes through a fluidised/aerated mass of microspheres (preferably electrostatically charged for uniform coating) in a through passage. The microspheres adhere to the resin coating and the coating is UV cured.
A problem with this known arrangement is that the fibre inlet and outlet can become blocked with particles requiring downtime to clear the blockage. This can be a particular problem when the microspheres are electrostatically charged as they are attracted to other surfaces than the fibre unit surface.
U.S. Pat. No. 5,851,450 describes an arrangement in which turbulence is introduced in the through passage to obtain an even distribution of microspheres in the resin coating. The microspheres enter through a horizontal duct, are deflected downwardly to introduce turbulence and further turbulence is induced by angled ribs in the through passage. In addition the fibre inlet and outlet are shaped so as to deflect the particles from the inlet and outlet. A problem with this arrangement is that it is difficult to adjust the operating properties of it without effectively rebuilding the apparatus as its operation is highly dependent on the configuration of the through passage.
A further problem with known arrangements is that the throughput speed for coating the fibre is limited, with the apparatus unable to reach speeds of the order of 300 m/min without severely affecting the uniformity of distribution of the embedded microspheres.
Further problems with known arrangements are that in some cases it may be difficult to ensure that there is no leakage of microspheres from known apparatus.